The present invention generally relates to optical element fabrication methods, and more particularly to a fabrication method for a transmission type optical element including a thin film. The present invention is suitably applicable for fabricating fluoride crystal materials such as calcium fluoride, magnesium fluoride, etc. suitable for various kinds of optical elements, lenses, window materials, prisms, etc. employed in a wide wavelength range, e.g., from the vacuum ultraviolet region to the far-infrared region.
Fluoride crystal has distinguished characteristics, which other optical materials do not have, such as less dispersion for white light, high transmittance for light having a wavelength shorter than the ultraviolet region, etc., thus having been so far used for a telescope lens with high resolution and low aberration. Recently, applications to a lens in an illumination optical system and a projection optical system in an exposure apparatus have been reviewed which uses, for a light source, light with a shorter wavelength such as KrF excimer laser with a wavelength of about 248 nm, ArF excimer laser with a wavelength of about 193 nm, F2 excimer laser with a wavelength of about 157 nm, etc. However, the shorter the wavelength becomes, the larger light's absorption, diffusion, and interference become, and thus, along with a shorter wavelength of light from a light source, the reflection of light on the surface of an optical element such as a lens and a mirror cannot be neglected.
In general, in an attempt to use an optical substrate for an optical system, anti-reflection film coating is widely applied onto an optical element for the purpose of increasing the transmittance of the optical element by decreasing its reflectance, and for the purpose of removing flare and ghost caused by the surface reflection of the optical system.
An optical thin film design defines a configuration of the anti-reflection film. Dielectric of an oxide or fluoride substance is used for a film material. As a manufacturing method of the anti-reflection film, there are a sputtering process that collides ion in plasma against a target surface so as to drive out target atoms to form a desired thin film onto a substrate by colliding, a chemical vapor deposition (“CVD”) method that uses a chemical reaction in a high-temperature space or space activated by plasma, light, etc. to form a desired thin film, a vacuum evaporation method that heats and evaporates a thin film substance using an electron beam and a heater, and so on to form a desired thin film onto a substrate.
However, if a thin film is formed onto a substrate by using the sputtering and CVD methods that use plasma, and by the vacuum evaporation method that uses an electron gun, an electric potential difference is produced among ion, electron and the substrate, and the ion and electron breaks into the substrate, thus damaging the substrate and the thin film formed on the substrate.
This damage would change a compositional ratio of chemical compounds, and impair crystallization, thereby deteriorating optical characteristics of an optical element. This damage includes generation of optical absorption (the so-called color center that absorbs light). For example, when calcium fluoride is used as a substrate, it is structured by electric coupling of Ca2+ and F− since calcium fluoride is an ionic crystal. Here, assuming F− is missing, and it is in an electrically unstable and high-temperature state, e− (electron) would occupy the place that lacks F− for electric stability, thereby producing the optical absorption. Then, although it is electrically stable, it is hard for light to transmit it, thus reducing its transmittance. In particular, optical absorption is noticeably observed in a light wavelength range equal to or shorter than 300 nm, thus presenting the problem that a transmission type optical element reduces its transmittance, being unable to obtain expected optical characteristics. This color center is inclined to appear when an ionically coupled substance is used for a substrate or as a material for a thin film.